Method for the production of tetrahydrofuran copolymers

ABSTRACT

The present invention provides a process for preparing polyoxyalkylene glycols of a certain molecular weight in one stage by copolymerizing tetrahydrofuran and alpha,omega-diols as the comonomer in the presence of a heteropolyacid and of a hydrocarbon, by distilling off a mixture of water and this hydrocarbon from the copolymerization, which comprises terminating the polymerization when this molecular weight is attained by adding water, comonomer, butanediol or butanediol-water mixtures.

CROSS REFERENCE TO RELATED APPLICATION

The present application is a National Stage application ofPCT/EP2003/014284, filed Dec. 16, 2003, which claims priority fromGerman Patent Application No. DE 102 59 136.9, filed Dec. 18, 2002.

TECHNICAL FIELD

The present invention relates to a novel process for preparingpolyoxyalkylene glycols (polyalkylene ether glycols) by copolymerizingtetrahydrofuran and alpha,omega-diols in the presence of aheteropolyacid and of a hydrocarbon, by distilling off a mixture ofwater and this hydrocarbon from the copolymerization and, after completecopolymerization, adding butanediol, butanediol-water mixture and/orcomonomer.

BACKGROUND

Polyoxyalkylene glycols are important starting materials for theproduction of elastic fibers, elastic construction materials andcoatings. They are prepared by polymerizing tetrahydrofuran (referred tohereinbelow as “THF”) or by copolymerization of THF with oxiranes suchas ethylene oxide or propylene oxide or with alpha,omega-diols in thepresence of cationic catalysts. EP-A 126 471, for example, discloses theuse of heteropolyacids as catalysts. This process makes polyalkyleneether glycols accessible in one stage, whereas other processes initiallyprovide the esters of the polyoxyalkylene glycols which still have to behydrolyzed to the polyoxyalkylene glycols before their use in the fieldof polymers.

The determination of a certain target molecular weight in theheteropolyacid-catalyzed THF polymerization by controlling the amountof, and appropriately metering in the amount present in thepolymerization system of, proton-donating compounds such as water in thecourse of the polymerization via measurement of the electricalconductivity is disclosed by DE-A 41 08 047.

A batchwise process for preparing THF copolymers using alpha,omega-diolsin the presence of a heteropolyacid is disclosed by JP-A 10-87811. Inthis process, a portion of the copolymerizetion solution is continuouslywithdrawn from the polymerization reactor and subjected to a process forwater removal. After 12 h, the polymerization is stopped. For therepetition of the experiment four times as described, fresh catalyst isused each time. This is uneconomic, since fresh catalyst has to be driedfor each new polymerization.

It has been found that the heteropolyacid phase of the copolymerizationmixture changes depending on the phase separation time and duration forthe recycling. Viscosity and color increase, which affects firstly thequality of the polymerization product, for example the color of the endproduct, and secondly the properties of the heteropolyacid, for examplepumpability, emulsifiability, on-stream time, but also the catalyticproperties.

It is an object of the present invention to make the copolymerization ofTHF with alpha,omega-diols in the presence of heteropolyacids moresimple and economic by providing copolymers of a certain molecularweight and finding a means of repeatedly using and recycling thecatalyst.

The novel process should additionally provide polyoxyalkylene glycolswith incorporation rates of the diol comonomer of from 5 to 40% byweight, based on the copolymer.

SUMMARY OF THE DISCLOSURE

We have found that this object is achieved, surprisingly, by a processfor preparing polyoxyalkylene glycols of a certain molecular weight inone stage by copolymerizing tetrahydrofuran (THF) and alpha,omega-diolsas the comonomer in the presence of a heteropolyacid and of ahydrocarbon, by distilling off a mixture of water and hydrocarbon fromthe copolymerization, which comprises terminating the polymerizationwhen this molecular weight is attained by adding water, comonomer,butanediol or butanediol-water mixtures.

The novel process allows THF copolymers of a certain molecular weight tobe prepared simply and reliably. The use of water, comonomer, butanediolor butanediol-water mixtures to terminate the copolymerization makespossible not only synthesis of THF copolymers of certain molecularweights, but results in a heteropolyacid-containing catalyst phase whichcan be reused, for example by recycling. The termination according tothe invention of the copolymerization results in an effectivelystabilized heteropolyacid which remains colorless, stable andcatalytically active even on prolonged intermediate storage.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graphical representation of electrical conductivity valuecorrelated with the average molecular weight of the copolymer beingformed for the dodecatungstophosphoric acid/neopentylglycol/THF/water/hexane reaction system.

DETAILED DESCRIPTION OF THE DISCLOSURE

According to the invention, the copolymerization is terminated by addingfrom 0.1 to 20% by weight of water, comonomer, butanediol and/orbutanediol-water mixture, preferably from 0.1 to 10% by weight, morepreferably from 0.1 to 5% by weight, based in each case on the totalamount of tetrahydrofuran, comonomer and heteropolyacid already used forthe copolymerization before the termination of the copolymerization onattainment of the desired molecular weight of the target polymer.Preference is given to adding comonomer, butanediol, butanediol-watermixture, particular preference to adding butanediol or butanediol-watermixture. This allows the electrical conductivity of the copolymerizationmixture to be correlated to the average molecular weight of the polymerforming. The possibility therefore exists of targeted termination of thecopolymerization on attainment of a certain conductivity value todetermine the average molecular weight of the copolymer being formed ina targeted manner while maintaining a narrow molecular weightdistribution.

The measurement of the electrical conductivity can, for example, becarried out in the process according to the invention with the aid oftechniques, circuits and measuring arrangements as described by T. andL. Shedlovsky in A. Weissberger, B. W. Rossiter (Ed.) Techniques ofChemistry, Volume I, pages 163-204, Wiley-Interscience, New York, 1971.Useful conductivity measuring instruments and conductivity measuringcells which can be used successfully in the process according to theinvention also include the commercially available instruments andelectrodes. The measuring electrodes used may be the customary platinumelectrodes.

Prolonged operation may result in the electrodes becoming coated in thecourse of time with polymer or by-products of the polymerizationreaction and the measurements thus being distorted. It is thereforeappropriate to check the function of the electrodes and to clean theelectrodes when required.

The conductivity can be measured in the homogeneous reaction mixture,the catalyst phase or else in the organic phase. The latter twopossibilities would arise when the reactor comprises a rest zone whichallows phase separation.

The electrical conductivity value can be correlated with the averagemolecular weight of the copolymer being formed, but is stronglytemperature-dependent and dependent on the organic hydrocarbon(azeotroping agent) used in each case. Taking into consideration theparticular heteropolyacid used, the comonomer used and thepolymerization temperature employed, this results in a virtually linearrelationship between the electrical conductivity measured and theaverage molecular weight of the polymer formed. Such a relationship isillustrated by way of example in FIG. 1 for the dodecatungstophosphoricacid/neopentyl glycol/THF/water/hexane reaction system at apolymerization temperature of 60° C.

In the process according to the invention, by which THF copolymers ofaverage molecular weight from 1000 to 2800 are obtainable, this means atermination of the copolymerization at a conductivity between 0.1 and 5μS, preferably between 0.1 and 3 μS and more preferably between 0.1 and2.5 μS. For better stabilization of the organic product phase beforeoxidative damage, 10-500 ppm, more preferably 50-300 ppm, of a radicalscavenger can be added to it. A particularly suitable radical scavengeris 250 ppm of 2,6-di-tert-butyl4-methylcresol (BHT).

In this application, the term “average molecular weight” or “averagemolar mass” refers to the number-average molecular weight M_(n) of thecopolymers present in the polymer formed.

The catalyst phase resulting from phase separation after termination ofthe copolymerization is stable and can be stored for a prolonged periodand if required reused for further copolymerizations. A catalyst phaseobtained from a preceding experiment can be reused in the nextcopolymerization experiment, but before reuse has to be freed of water,comonomer, butanediol or butanediol-water,mixture added for termination.This can be effected, for example, by distillation.

The copolymerization of THF with alpha,omega-diols releases water ofreaction. Since water firstly adversely affects the catalyst activityand secondly acts as a chain-termination reagent (known as a “telogen”),it is necessary to remove the water of reaction and also the water ofcrystallization from the copolymerization stage to achieve a certainmolecular weight.

The hydrocarbons used in the process according to the invention shouldbe suitable for azeotrope formation with water. Useful hydrocarbons are,for example, aliphatic or cycloaliphatic hydrocarbons having from 4 to12 carbon atoms or aromatic hydrocarbons having from 6 to 10 carbonatoms or mixtures thereof. Specific mention is made, for example, ofpentane, hexane, heptane, octane, decane, cyclopentane, cyclohexane,benzene, toluene, xylene or naphthalene, and among these preference isgiven to pentane, cyclopentane, hexane and octane, and particularpreference to pentane.

The hydrocarbons are added to the copolymerization mixture (consistingof THF and neopentyl glycol (NPG)) at the beginning of the reaction inan amount of from 1×10⁻⁴% by weight (corresponding to 1 ppm) to 30% byweight, based on the reaction mixture composed of alpha,omega-diol andTHF, preferably from 1 ppm to 16% by weight, more preferably from 1 ppmto 10% by weight. However, it is also possible to introduce thehydrocarbon into the top of the distillation column to remove themixture of hydrocarbon and water. The total amount of water which isdischarged from the copolymerization can be used to adjust theparticular molar mass.

Useful comonomers are alpha,omega-diols apart from 1,4-butanediol, forexample C₂- to C₁₀-alkanediols such as ethylene glycol, propyleneglycol, 1,3-propanediol, 1,3-butanediol, 2-methylbutanediol,1,5-pentanediol, 1,6-hexanediol, neopentyl glycol,3-methyl-1,5-pentanediol, 1,8-octanediol, 1,10-decanediol, diethyleneglycol, triethylene glycol, low molecular weight THF copolymers with thealpha,omega-diols mentioned here having an average molecular weight offrom 200 to 600 dalton or mixtures thereof. The comonomers used arepreferably low molecular comonomers having an average molecular weightof from 200 to 600 dalton and neopentyl glycol, more preferablyneopentyl glycol. For the purposes of this invention, 1,4-butanediol isnot a comonomer, since it leads to the polytetrahydrofuran homopolymerand does not give a copolymer.

It is also possible to use mixtures of tetrahydrofuran, water and2-methylbutanediol, and the proportion of 2-methylbutanediol in suchmixtures may be between 100 ppm and 60% by weight, based on the mixture.

According to the invention, from 1 to 60% by weight of thealpha,omega-diol, based on the tetrahydrofuran used, preferably from 2to 40% by weight, more preferably from 3 to 20% by weight, are used inthe copolymerization.

The THF is copolymerized with alpha,omega-diols in the presence ofheteropolyacids as catalysts in a manner known per se, as described, forexample, in EP-A 126 471.

Heteropolyacids which are used in accordance with the invention areinorganic polyacids which, in contrast to isopolyacids, have at leasttwo different central atoms. Heteropolyacids are formed from polybasicoxygen acids, each of which are weak, of a metal such as chromium,molybdenum, vanadium and tungsten, and also of a nonmetal such asarsenic, iodine, phosphorus, selenium, silicon, boron and tellurium, aspartial mixed anhydrides. Examples include dodecatungstophosphoric acidH₃(PW₁₂O₄₀) or decamolybdophosphoric acid H₃(PMo₁₂O₄₀). As the secondcentral atom, the heteropolyacids may also contain actionides orlanthanoids (see Z. Chemie 17 (1977), pages 353 to 357 or 19 (1979),308). The heteropolyacids can be generally described by the formulaH_(8-n)(Y^(n)M₁₉O₄₀) where n=valency of the element Y (for exampleboron, silicon, zinc) (see also Heteropoly- and Isopolyoxometalates,Berlin; Springer 1983). Catalysts which are particularly suitable forthe process according to the invention are phosphotungstic acid,phosphomolybdic acid, silicomolybdic acid and silicotungstic acid.

The heteropolyacids used as catalysts in the copolymerization may beused either dried (from 1 to 10 mol of water/mol of heteropolyacid) orundried (from 10 to 40 mol of water/mol of heteropolyacid).

The water present in the copolymerization reactor which is partly waterof crystallization from the heteropolyacid and partly water formedduring the reaction is removed with the aid of a customary distillationapparatus directly from the copolymerization, i.e. from thecopolymerization reactor without intermediate work-up steps such asphase separations, as a mixture with the hydrocarbon added with thefresh feed with water, at a temperature of from 40 to 120° C., morepreferably from 50 to 70° C., and a pressure of from 150 mbar to 2 bar,preferably 230 mbar.

The vapors formed are preferably condensed in a surface condenser;however, quenches and injection condensers are also possible. Theresulting condensate is fed to solvent work-up to separate the water. Itis particularly advantageous to at least partly recycle the condensateinto the reactor, i.e. to remove the heat of reaction by means ofevaporative cooling. To achieve very high water contents in thecondensate to be removed, another multistage countercurrentrectification column charged with the recycle condensate as reflux canbe inserted between reactor and condenser.

In a further embodiment, THF is distilled off at the same time as themixture of the hydrocarbon used in the copolymerization with water,which, depending on the hydrocarbon, may form a ternary azeotrope.

The hydrocarbon distilled off in a mixture with water or the mixtures ofwater and hydrocarbon with tetrahydrofuran may be dried with a suitablesolid adsorbent, for example over molecular sieves, and recycled intothe copolymerization. Phase separation into an aqueous phase and ahydrocarbon phase is also conceivable. The aqueous phase contains up to5% by weight of THF, preferably <1% by weight. It also contains therespective hydrocarbon in concentrations of <1% by weight. THF and thehydrocarbon may be recovered by distillative work-up of the aqueousphase and recycled. However, the aqueous phase may also be discarded.

In the copolymer solution (containing THF, heteropolyacid and copolymer)remaining after removal of the hydrocarbon/water mixture, theconductivity is determined until the desired value has been attained.The copolymerization is then terminated by adding the amount of water,comonomer, butanediol and/or butanediol-water mixture required inaccordance with the invention. Afterwards, the copolymer solution ispreferably transferred to a phase separator. By adding further amountsof hydrocarbon, the heteropolyacid is removed from the product phase.This process known per se, for example from EP-A 181 621, leads topostprecipitation of the heteropolyacid from the organic phase. Thehydrocarbon used is preferably the hydrocarbon already used in thecopolymerization.

The upper organic phase contains the majority of the copolymer and THFand also smaller residual amounts of heteropolyacid or its subsequentproducts. Their content generally does not exceed 0.03% by weight, basedon the copolymerization effluent. Nevertheless, it has been recognizedthat these residual amounts of the catalyst and its subsequent productshave to be removed, since they adversely affect the properties of thecopolymers for their further use.

The THF can be distillatively removed from the copolymer before or afterthe removal of the catalyst fractions and/or subsequent catalystproducts by filtration, for example ultrafiltration, adsorption on solidadsorbents and/or with the aid of ion exchangers, although preference isgiven to filtration and adsorption on solid adsorbents. Preference isgiven to filtering without preceding removal of the THF by distillation.

The adsorption on the solid adsorbents mentioned may also be combinedwith a neutralization of the polymerization effluent by bases. Usefulbases include, for example, the hydroxides and carbonates of the alkalimetals and alkaline earth metals.

The adsorption is effected preferably on activated carbon and/or metaloxides and/or ion exchangers, at temperatures of from 10 to 75° C.,preferably from 20 to 70° C. Particular preference is given to effectingthe removal in work-up stage a) on ion exchangers and/or activatedcarbon. Preferred metal oxides are sodium hydroxide, aluminum oxide,silicon dioxide, titanium dioxide, zirconium dioxide, lanthanum oxideand/or calcium oxide.

Suitable activated carbon may, for example, be obtained from Merck,Darmstadt, or in the form of the commercially available CPG UF 8×30activated carbon from Chemviron Carbon.

Suitable ion exchangers are, for example, anion exchangers such as thecommercially available Lewatit® MP 600R which may be obtained from BayerAG, Leverkusen, mixed ion exchangers, for example the commerciallyavailable SerdolitR® which may be obtained from Serva, Heidelberg, ormolecular sieves having pore sizes of from 3 to 10 Å.

The removal according to the invention of the catalyst fractions and/orsubsequent catalyst products by adsorption on solid adsorbents ispreferably carried out in a fixed bed at an hourly space velocity ofgenerally from 0.2 to 5 kg/l*h, in particular from 0.4 to 4 kg/l*h (kgof polymerization effluent per liter of adsorbent per hour).

The process according to the invention may be carried out batchwise,semibatchwise or continuously. The preferred variant is batchwise.

In the case of the continuous method, fresh THF monomer is metered intothe reactor via a fill level regulator in the presence of water (0.1-5%by weight, preferably 0.1-3.5% by weight, more preferably 0.1-2% byweight, of THF, based on the total amount of THF monomer and comonomer,for example neopentyl glycol). Advantageously, fresh feed solution isfed at the rate at which product and unconverted monomer are dischargedfrom the reaction apparatus. In this way, the residence time, andtherefore the polymerization time, can be controlled, which provides afurther means of influencing and adjusting the average molecular weightof the polymer being formed. Depending on the amount of catalyst and thereaction temperature in the batchwise process, the copolymerization isgenerally carried out over a period of from 0.5 to 70 hours, preferablyfrom 5 to 50 hours and more preferably from 10 to 40 hours. In the caseof the continuous process, residence times of from 1 to 50 hours andpreferably from 10 to 40 hours, are typically set. At the beginning of acontinuous reaction, the reaction system described requires a certainamount of time until a steady-state equilibrium has been established,and during this time it may be advantageous to keep the reactor outletclosed, i.e. to discharge no product solution from the reactionapparatus. The conductivity can be measured continuously with the aid ofa conductivity measuring cell disposed in the copolymerization solution.

For the batchwise, semibatchwise and continuous method, theheteropolyacid is advantageously used in amounts of from 1 to 300 partsby weight, preferably from 5 to 150 parts by weight, based on 100 partsby weight of the monomers used (THF monomer and alpha,omega-diolcomonomer). It is also possible to add relatively large amounts ofheteropolyacid to the reaction mixture.

The heteropolyacid can be fed to the reaction in solid form, whereuponcontacting with the further reactants results in it gradually going intothe liquid catalyst phase. Another procedure is to slurry the solidheteropolyacid with the alpha,omega diol to be used and/or the THF andpass the resulting catalyst solution into the reactor as a liquidcatalyst phase. Even the catalyst phase or the monomeric startingmaterial can be initially charged in the reactor. However, bothcomponents can also be introduced into the reactor at the same time.

The copolymerization is typically carried out at temperatures of from 20to 100° C., preferably from 30 to 80° C. Preference is given to workingunder atmospheric pressure, although reaction under pressure, especiallyunder the autogenous pressure of the reaction system, may equally proveconvenient and advantageous.

In the batchwise, semibatchwise method and the continuous procedure, thereactors should be equipped with efficient mixing apparatus, for examplestirrers.

Suitable reactors are any reactors which are known to those skilled inthe art and have internal and/or external free liquid surface area forthe necessary vaporization of the water-containing vapors in whichsufficiently high shear forces are achieved in the liquid to suspend thecatalyst phase in the homogeneous monomer/polymer phase (stirred tanks,circulation reactors, jet loops, pulsed internals). A particularlyadvantageous design is the configuration as a jet loop, since thenecessary heating of the reactor can be integrated here in a simplemanner into the liquid circulation stream. The water-containing mixtureof the hydrocarbon is evaporated out of the reaction mixturecontinuously or batchwise and the water content of the reactor contentsis thus set to values advantageous from the point of view of thereaction.

The process according to the invention is advantageously carried outunder an inert gas atmosphere, and any desired inert gases such asnitrogen or argon can be used. The reactants are freed before use of anywater and peroxides contained therein.

The reaction can be performed in conventional reactors or reactorarrangements suitable for continuous processes, for example in tubularreactors which are equipped with internal fitments which ensure goodmixing of the emulsion-like copolymerization mixture, or else in stirredtank batteries.

The process according to the invention can provide polyoxyalkyleneglycols, in particular copolymers of THF and neopentyl glycol,economically and in good yield, selectively and with a narrow molecularweight distribution, and also in pure form. The copolymers haveincorporation rates of the alpha,omega-diol comonomer of from 10 to 50%by weight, based on the copolymer, and average molecular weights M_(n)of from 600 to 6000. The polyoxyalkylene glycols which can be preparedaccording to the invention find use, for example, for preparing specialpolyurethanes which are suitable as highly elastic composite materials.A polyurethane polymer which contains the copolymers which can beprepared in accordance with the invention has a high elongation atbreak, a small change in the stress on elongation, a small hysteresisloss on expansion and contraction and also a high elasticity even inextreme cold.

The invention is illustrated by the examples which follow.

EXAMPLES

Determination of the Color Number

The polymers freed of solvent are analyzed untreated in a Dr. Lange LICO200 calorimeter. 100-QS precision cuvettes (layer thickness 50 mm,Helma) are used.

Determination of the OH Number

The hydroxyl number is that amount of potassium hydroxide in mg which isequivalent to the amount of acetic acid bound in the acetylation of 1 gof substance. The hydroxyl number is determined by the esterification ofthe hydroxyl groups present with an excess of acetic anhydride. Afterthe reaction, the excess acetic anhydride is hydrolyzed with water andback-titrated as acetic acid with sodium hydroxide solution.

Copolymerization Ratio Determination

The copolymerization ratio was determined by ¹H NMR using a Bruker dpx400 instrument; 400 MHz, log. Standard: tetramethylsilane (TMS) usingthe solvent CDCl₃.

To calculate the incorporation rate, the integrals I of the methyl groupsignals of neopentyl glycol (NPG) (0.8-1.1 ppm) and of the internal CH₂groups of the polytetrahydrofuran units (1.4-2.0 ppm) are used:

$\frac{{\,^{l}{NPG}} \times 4}{{\,^{l}{THF}} \times 6} = {{incorporation}\mspace{14mu}{rate}}$Determination of the Conductivity

Electrode: LTA 01 glass/platinum 2-electrode measuring cell, K approx.0.1 cm⁻¹; from Knick Conductometer (evaluation unit) WTW Knick 702

From the current measured, the measuring instrument initially calculatesthe conductance of the solution analyzed on the basis of Ohm's law and,taking into account the cell constant, the conductivity. The temperatureadjustment is effected manually on the evaluation unit.

Example 1

In a 2 l jacketed reactor equipped with a magnetic stirrer and attacheddistillation column (50 cm) with combined water separator, a mixture of800 g of THF, 48 g of neopentyl glycol and 50 g of pentane was stirredto a homogeneous solution. To this were added 197 g of hydrateddodecaphosphotungstic acid (H₃PW₁₂O₄₀*x H₂O where x=20-40 from Merck),with stirring. The temperature of the heating medium (oil) was set to94° C. The reaction temperature was held at 65° C. The conductivity atthe start of the copolymerization was 150 μS.

The THF/pentane/water mixture evaporating off during the reaction wasseparated in the column. The pentane/water mixture is taken off overheadand condensed in the water separator. The liquid phase of the columnconsists mainly of THF and is recycled into the polymerization stage.The pentane-water mixture separates into two phases, and the upperorganic phase runs back into the top of the column. The lower aqueousphase is discarded. During the reaction, 19 g of water were removed.

After 22 h, the reaction was terminated at a conductivity of 2.1 μS byadding 5 g of water and 250 ppm of 2,6-di-tert-butyl-p-cresol (BHT) and600 g of hexane. After completed phase separation, the lower aqueouscatalyst phase (236 g) is discharged and stored for three days.

The upper phase (809 g) was passed at 20° C. over a fixed bed chargedwith an anion exchanger (volume: 1 l) of the Bayer Lewatit® MP 600 Rbrand.

Afterwards, THF and heptane were removed on a rotary evaporator at 140°C. and a pressure of 20 mbar to obtain a copolymer having an OH numberof 60 mg of KOH/g of copolymer. The NPG incorporation rate was 11.4 mol%. Further data can be taken from Table 1.

Example 2

The aqueous heteropolyacid phase removed from Example 1 is used inExperiment 2. The initial conductivity is 42 μS. After a running time of18 h and a conductivity of 2.1 μS, the reaction is terminated. Thework-up delivers 236 g of an aqueous HPA phase. Further data are listedin Table 1.

Comparative Example C1

The aqueous heteropolyacid phase removed from Example 2 is used inComparative Example 1. The initial conductivity is 27 μS. After arunning time of 22 h and a conductivity of 2.1 μS, the reaction isterminated by switching off heater and stirrer.

The work-up delivers 240 g of an aqueous HPA phase which is stored forthree days. Further data are listed in Table 1. The recoveredheteropolyacid phase became solid after 3 days, could no longer berecycled and could no longer be used. The color of the heteropolyacidphase changed from colorless to dark blue.

Addition of HPA¹⁾ Running water at end Conduc- NPG THF NPG phase Temp.time of reaction tivity EVR OH incorp. Color Ex. [g] [g] [g] [° C.] [h][g] [μS] [%] No. [mol %] No. 1 800 48  197* 65 21 5 2.1 34 60 11.4 10 2800 48 236 65 18 10 2.2 32.5 62 11.8 9 C1 800 48 236 65 22 — 2.1 33.6 6312.2 10 ¹⁾HPA = Heteropolyacid EVR = Evaporation residue

Example 3

In a 1 l jacketed reactor equipped with a magnetic stirrer and attacheddistillation column (50 cm) with combined water separator, a mixture of590 g of THF, 30 g of neopentyl glycol and 60 g of hexane was stirred toa homogeneous solution. To this were added 150 g of hydrateddodecaphosphotungstic acid (H₃PW₁₂O₄₀*x H₂O where x =20-40 from Merck),with stirring. The temperature of the heating medium (oil) was set to94° C. The reaction temperature was held at 65° C.

The THF/hexane/water mixture evaporating off during the reaction wasseparated in the column. The hexane/water mixture is taken off overheadand condensed in the water separator. The liquid phase of the columnconsists mainly of THF and is recycled into the polymerization stage.The hexane-water mixture separates into two phases, and the upperorganic phase runs back into the top of the column. The lower aqueousphase is discarded. During the reaction, 20.7 g of water were removed.

After 19 h, the reaction was terminated at a conductivity of 3.2 μS byadding 5 g of water and 250 ppm of 2,6-di-tert-butyl-p-cresol (BHT) and600 g of hexane. After completed phase separation, the lower aqueouscatalyst phase (208 g) is discharged and stored for three days.

The upper phase (475 g) was passed at 20° C. over a fixed bed chargedwith an anion exchanger (volume: 1 l) of the Bayer Lewatit® MP 600 Rbrand.

Afterwards, THF and heptane were removed on a rotary evaporator at 140°C. and a pressure of 20 mbar to obtain a copolymer having an OH numberof 70 mg of KOH/g of copolymer. Further data can be taken from Table 2.

Example 4

The aqueous heteropolyacid phase removed from Example 3 is used inExperiment 4. After a running time of 24 h and a conductivity of 3.6 μS,the reaction is terminated by adding 5 g of water. The work-up delivers199 g of an aqueous HPA phase.

Comparative Example 2

The aqueous heteropolyacid phase removed from Example 4 is used inComparative Example 2. After a running time of 25 h and a conductivityof 2.3 μS, the reaction is terminated without adding water by switchingoff the heating. The work-up delivers 170 g of an aqueous HPA phase.

The recovered heteropolyacid phase became solid after 3 days, could nolonger be recycled and could no longer be used. The color of theheteropolyacid phase changed from colorless to dark blue.

Further data are listed in Table 2.

Addition of HPA¹⁾ Running water at end Conduc- THF NPG phase Temp. timeof reaction tivity EVR OH Ex. [g] [g] [g] [° C.] [h] [g] [μS] [%] No. 3590 30  150* 65 19 5 3.2 30.0 70 4 590 30 199 65 24 5 3.6 29.6 75 C 2590 30 169 65 25 — 2.3 33.4 58 ¹⁾HPA = Heteropolyacid EVR = Evaporationresidue

1. A process for preparing polyoxyalkylene glycols comprising copolymerizing, in one stage, tetrahydrofuran and alpha, omega-diols with the exception of butanediol as the comonomer in the presence a heteropolyacid and of a hydrocarbon, distilling off a mixture of water and the hydrocarbon from the copolymerization, and terminating the polymerization by adding water when a molecular weight of from 1,000 to 2,800 is attained.
 2. The process as claimed in claim 1, wherein between 0.1 and 10% by weight of water, based on the total amount of tetrahydrofuran, comonomer and heteropolyacid already used for the copolymerization, is added.
 3. The process as claimed in claim 1, wherein the attainment of the molecular weight is determined by measuring the electrical conductivity of the copolymerization mixture.
 4. The process as claimed in claim 3, wherein the water is added at a conductivity of from 0.1 to 5 μS.
 5. The process as claimed in claim 1, wherein the alpha, omega-diol used is neopentyl glycol.
 6. The process according to claim 2, wherein the attainment of the molecular weight is determined by measuring the electrical conductivity of the copolymerization mixture.
 7. The process according to claim 6, wherein the water is added at a conductivity of from 0.1 to 5 μS.
 8. The process according to claim 2, wherein the alpha, omega-diol used is neopentyl glycol.
 9. The process according to claim 3, wherein the alpha, omega-diol used is neopentyl glycol.
 10. The process according to claim 4, wherein the alpha, omega-diol used is neopentyl glycol. 